This invention relates to continuous tape having an adhesive on one surface, useful for sealing boxes or the like.
Coatings of an adhesive in liquid form are conventionally applied to a continuous web or substrate made of a flexible material. For example, many box sealing tapes in commercial use are made by applying a pressure-sensitive acrylic adhesive onto a polypropylene web.
The most widely used coating technique involves use of a wire-wound metering rod, also called a Mayer rod, doctor rod or applicator rod. The rod has a support core that is typically a stainless steel rod between about 0.15 inch (in.) to about 1 in. (about 3.8 to about 25 mm) in diameter, 0.25 in. (6.4 mm) diameter being typical, that is wound with a tightly wrapped spiral of wire, also typically stainless steel. The diameter of the wire is selected based on the coating thickness desired, e.g., one rule of thumb being a 1:10 ratio between the coating thickness and wire diameter so that a wire of 0.02 in. (0.51 mm) diameter on a wire-wound metering rod would produce a wet film coating of about 0.002 in. (2 mils or 0.051 mm) thickness.
In a coating apparatus, the moving continuous substrate web is passed via a conveyor system of rolls or rollers through a wetting station where the web is contacted on one surface with the wet adhesive, e.g., using an applicator roller coated with the adhesive or by immersion of the web into a tank containing the adhesive. The gross coated web is then passed to a metering station where the moving web passes above and contacts the wire-wound metering rod. This allows only a measured amount of adhesive coating to remain on the web, the excess liquid adhesive being returned by gravity to a reservoir of adhesive. The wire-wound metering rod is normally rotated, so that the area of contact of the metering rod moves either in the same direction or in the opposite direction to that of the web travel.
Although wire-wound metering rods provide a high degree of precise metering of liquid coating material onto the moving continuous web, this system typically results in an uneven adhesive thickness at the two edges of the coated web, at both ends of the wire-wound metering rod. Rubber wiper blades on the edges of the adhesive applicator roll have been typically used to prevent the coating from wrapping around the edge of the web and contaminating the uncoated side, but these are not effective in preventing adhesive edge beading or buildup.
In commercial practice, adhesive tape is made from wide webs are used in a full web coating process. The adhesive coated web is wound, after the adhesive is dried, on a support cylinder, e.g., a hollow cardboard cylinder, and the finished, wound reel is cut into appropriate widths, e.g., between 0.5 to 5 in. (12.7 to 127 mm), to make many rolls of wound adhesive tape. The lateral edge portions of the continuous wide web, which contains both uncoated web at its edge and the edge portion of the adhesive coating with the coating bead edge or uneven coating thickness, are normally slit off and discarded.
This edge slitting practice is necessary because, if the edge portion were to be wound onto the finished roll of coated tape, the presence of the edge buildup or edge unevenness would result in an unevenly wrapped roll. This discarded edge waste portion represents a significant added cost to the finished product, since the raw materials (i e., film substrate and adhesive) on the discarded waste portion cannot be recycled. Despite the longstanding existence of this problem and economic incentive for solving it, the box sealing tape industry has failed to develop a means for precisely metering a uniform thickness of a liquid coating of adhesive at the edge of the applied coating, without edge beading or buildup.
The problem of edge beading or buildup also exists with attempts to create adhesive-free stripes on sealing tapes of the type disclosed in U.S. Pat. No. 5,366,775 of Kao, which describes an adhesive tape with an adhesive-free strip-like area in the middle of the tape to facilitate easy peeling from the roll. U.S. Pat. No. 2,819,180 of Koenig describes a self-adhesive tape for specialty applications having two adhesive strips running in a longitudinal direction. No information is provided in either patent about how such tapes may be manufactured.
The present invention solves the problem of adhesive edge beading or buildup in tape manufacture and provides a method of manufacturing an economical grade of box sealing tape that has satisfactory sealing characteristics but that contains substantially less adhesive than conventional box sealing tapes.